1. Field of the Invention
The invention relates to a process for the homogeneous incorporation of a stabilizer into a finely divided solid polycondensate.
2. Discussion of the Background
Polycondensates are widely employed for many purposes. During processing and use, they are exposed to the effects of air and light. This causes them to discolor, while their mechanical properties simultaneously deteriorate. This effect occurs all the more, the higher the temperature. To avoid the deficiencies above-mentioned, the polycondensates must be stabilized.
The most usual stabilizer incorporation process starts from a polymer granule/stabilizer mixture. This is melted and homogenized in a kneader or extruder. The melt is then forced through a perforated plate, and the strands are cooled in water and granulated. However, the process is expensive since it requires special kneaders and extruders with corresponding metering devices, and in general takes two granulating and two drying steps in going from the monomer to the stabilized polymer product.
It is also known to add stabilizers to the monomers in the reactor and to carry out the polycondensation in their presence. The disadvantage here is that the heat-sensitive stabilizers decompose during a long residence time and at a high temperature. The stabilizers sometimes can also react with the polymer, some of the stabilizer molecule becoming bonded to the polymer chain. The activity of the stabilizers is partially lost in this way.
DE-A 21 53 249 describes that improved polyamides are obtained by shortening the residence time of the stabilizer under heat. In this procedure, stabilizers are stirred into the polyamide melt at temperatures below 270.degree. C. after the polycondensation. However, this discontinuous process cannot be satisfactory on a production scale since in the temperature range from 220.degree. to 270.degree. C., in which the polyamide melts are stirrable, the stabilizers undergo decomposition, transamidation and transesterification reactions during the 5- to 8-hour melt discharge phase, leading to significant losses in quality with respects of homogeneity, color and stability to ageing.
Another form of incorporation in the presence of a polyalkenamer is described in DE-A 36 13 528 (U.S. Pat. No. 4,840,993). In this case, liquid auxiliaries and additives are incorporated at a temperature of not more than 70.degree. C. However, the process described cannot be applied to polyalkenamer-free polymers.
The object of the present invention is to provide a diverse incorporation process without using a kneader or extruder, by which homogeneous distribution of the stabilizer in the polymer is achieved under mild conditions.